Camera systems often use CCD image sensors for reasons of better image quality, in particular with respect to noise and dynamic range, when compared to other image capturing methods. Current developments on CMOS image sensors show improvements in this type of sensors. Further, CMOS sensors have significant advantages in production, as they can be made using the same techniques that are used for signal processing. This allows for integrating the image sensor and at least part of the signal processing circuitry into one device, thus bringing significant reductions in costs. Further, CMOS image sensors can provide higher field or frame rates, which is important for capturing fast movements.
CMOS image sensors for digital camera applications are generally designed and produced following standard CMOS processes. Additional pixel process steps are added. The use of analogue IPs, or building blocks or models, in an IC design like a CMOS image sensor for digital camera applications can significantly shorten the development time and reduce development costs compared to customized circuit design.
Process variances and mask tolerances are the main reason for mismatches in the performance and electrical behaviour between pixel cells of one sensor. Known effects resulting thereof are, inter alia, the variation of the dark voltage, or of the reset voltage of the pixel. The dark voltage or reset voltage is the voltage level a pixel assumes after a reset pulse charges the capacitive node of its photodiode to a reset level e.g. a high level. Starting from that voltage the capacitive node is discharged by the photodiode during the exposure time. The voltage at the end of the exposure time is called the bright or video voltage and corresponds to the illumination of the pixel. The absolute level of this voltage is correlated with the dark level of the pixel at the beginning of the exposure time. It is to be noted that the term voltage is used interchangeably with the term signal throughout this specification, unless otherwise indicated.
CMOS imagers use analogue-to-digital converters, or A/D-converters, for converting an analogue signal into a digital signal. Standard IPs or building blocks for A/D converters are usually adapted to an input voltage range which is fully differential. The term “fully differential” is used in the sense that the positive and also the negative input of the differential A/D converter may vary between the same high voltage and low voltage limits independently from each other. That is to say, differential A/D converters can also accept an inverted signal, in which the signal at the negative input is higher than that at the positive input. The full resolution at the output of the ADC can only be achieved when the full differential voltage range is used at the inputs.
If a CMOS image sensor pixel cell is reset after illumination, its output voltage is set to the reset level, corresponding to the dark value of the pixel. The reset level typically is a high level compared to the voltage level of a fully exposed pixel. The reset level is stored in a capacitance, which may also be a parasitic capacitance or a blocking layer capacitance of a p-n-junction. After exposure of the light sensitive element of the pixel this voltage level is reduced to lower values proportional to the light intensity integrated during exposure, resulting in the bright value. These two output values, the dark value and the bright value of the pixel cell, are available for further signal processing. They are not fully differential, since the voltage corresponding to the dark value is always higher than or equal to the voltage corresponding to the bright value. It is recalled that fully differential in the sense of the invention corresponds to signals independently assuming values between the same high and low signal values. In state-of-the-art CMOS image sensors, as was stated above, the bright value is always tied to the dark value. Therefore, both signals are not independent from each other. As a result, only half of the voltage range of a standard differential amplifier or differential A/D converter can be used. The effective resolution is reduced by 2.